I imagine it would be difficult to measure unless you set up an aquarium for it. You would need to ensure there were no external influences for
long enough to see an effect and to find a safe concentration would require repeated trials. I imagine putting anything other than sea water onto
coral will change it somehow, and most man-made things introduced to reefs don't really help (except electricity maybe: Pemuteran, in NW Bali) and
injecting coral with 100% DEET probably won't do it any good. On the other hand, I just looked up some numbers on DEET. It doesn't readily dissolve
in water, and is less dense (0.995 vs 1.028) so it should float (picaridin too). If it comes off the skin and drifts into the reef, or is ingested
by some marine life while still in high concentration then it can make it into the reef system, otherwise it is likely to be dispersed at the
surface.
This doesn't prove anything, but I think when I visit in July I'll bring deet, picaradin and cactus juice and try them all, as I agree that they
work differently on each person, otherwise there would only be one. I'll try not to use deet before diving to at least avoid melting my 1mm suit.
Now, if only we could find an insect replant that was also an attractor for whale sharks in July
DEET:
Deet
Picaridin
http://www.epa.gov/pesticides/foia/reviews/070705/070705-2005-09-23a.pdf